The turtle bladder epithelial cell layer possesses the acetazolamide sensitive functions of electrogenic acidification and cAMP-dependent electrogenic alkalinization. Both of these occur independently from a third process of electrogenic Na-reabsorption. The epithelial cell layer is composed of two basic cell types: granular (G) cells, which contain large intracytoplasmic granules and comprise nearly 80% of the total epithelial cell population; and carbonic anhydrase rich (CA) cells, which are less numerous, and contain abundant carbonic anhydrase activity. The facts that the acid-base transport processes are sensitive to acetazolamide, and that the CA cells contain acetazolamide-sensitive carbonic anhydrase activity, leads to the conclusion that the CA cells are the source of those processes. The independence of those processes from Na-reabsorption leads to the additional conclusion that Na-reabsorption may be produced only by the G cells. By directing the impalement of apical membranes of either G or CA cells with potential sensing microelectrodes, it is planned to determine the cell type responsible for each of these three processes. In a second approach, it will be determined if in the living tissue, changes in acid-base transport result in concomitant morphological changes in epithelial cell morphology. The localization of the cell type responsible for the acid-base and Na-reabsorptive functions, and the determination of transmembrane electrical characteristics of acid-base transport, will provide new insights into how these processes arise and are regulated in both this tissue and perhaps in the mammalian nephron.